A VR motor is a motor in which excitation current is supplied to exciting coils of a stator so that salient-pole teeth of the stator are excited, a salient-pole tooth of a rotor is attracted by means of a magnetic force of attraction generated in the salient-pole teeth of the stator, and the rotor is rotated by means of the resulting rotatory force. This motor is provided with switching devices for supplying excitation current to the exciting coils for individual phases, and the switching devices are opened and closed in response to the rotational angle of the motor, whereby the exciting coils for the individual phases are excited to rotate the rotor.
In the case of a three-phase VR motor with A-, B- and C-phases, for example, an A-phase switching device is closed to connect an A-phase exciting coil and a DC power source, thereby starting to supply current. When an A-phase salient-pole tooth attracts the salient-pole tooth of the rotor so that the rotor rotates through a predetermined angle, the A-phase switching device is opened to suspend current supplying. Then, a B-phase switching device is closed to excite a B-phase exciting coil. The motor is rotated in one direction by successively exciting the A-, B-, and C-phase coils in a like manner, thereafter. In reversing the motor, the motor can be reversed by exciting the A-, C-, and B- phase coils in the order named.
In controlling the current flowing through each exciting coil of this VR motor according to the pulse-width modulation system (PWM system), driver circuits must be formed independently for the individual phases. Therefore, each phase requires four switching devices or a combination of two switching devices and two diodes. Thus, a driver circuit of the conventional VR motor requires use of a number of switching devices and diodes, so that the driver circuit itself is increased greatly in cost, and requires two cables for each phase. Accordingly, the driver circuit becomes more expensive, and its wiring entails more man-hours.
An arrangement improving this point has been disclosed in a patent application in Japan (Jpn. Pat. Appln. No. 4-84966). According to this patent application, there is provided a driver circuit which requires use of only (N+1) switching devices where N is the number of phases of the VR motor.
The circuit diagram of FIG. 2 shows an example of the driver circuit of the three-phase VR motor in which the number of switching devices-is equal to "phase number+1."
In FIG. 2, reference numeral 1 denotes a rectifier circuit which rectifies three-phase alternating currents R, S and T to generate a DC voltage (main voltage) V. C1 designates a smoothing capacitor. In this driver circuit, a common series circuit is formed such that one end of a common switching device Q1 for pulse width modulation (PWM) operation is connected to a positive terminal of the rectifier circuit 1, and the cathode of a diode D1 is connected to the other end of the device Q1, the anode of the diode D1 being connected to a negative terminal of the rectifier circuit 1.
Moreover, this driver circuit is provided with series circuits for the individual phases, that is, the A-, B-, and C-phases, such that one end of each of switching devices (transistors) Q2, Q3 and Q4 for alternatively exciting the A-, B-, and C-phase coils is connected to the negative terminal of the rectifier circuit 1, the other ends are connected individually to the respective anodes of diodes D2, D3 and D4. The respective cathodes of the diodes D2, D3 and D4 are connected to the positive terminal of the rectifier circuit 1.
The junctions of the switching devices Q2, Q3 and Q4 and the diodes D2, D3 and D4 of the series circuit for the individual phases are connected to one ends of their corresponding exciting coils of the reluctance motor, while the respective other ends of the exciting coils are connected to the junction of the switching device Q1 and the diode D1 of the common series circuit.
As described above, the switching devices in this driver circuit include a common one (common switching device Q1) and one for each phase.
In FIG. 2, symbols ZA, ZB and ZC designate the impedances of the A-, B-, and C-phase coils of the VR motor, respectively. Also provided are current detectors for detecting currents i.sub.a, i.sub.b and i.sub.c flowing through the individual coils. In the example shown in FIG. 2, current detecting resistors Ra, Rb and Rc are shown as detectors based on current detecting resistances. Symbol i.sub.t designates a total current given by i.sub.t =i.sub.a +i.sub.b +i.sub.c.
In connection with this arrangement, the drive of the VR motor, that is, excitation of the coils, taking the case of excitation of the A-phase coil, will be described.
(1) When a positive voltage is applied to the A-phase coil to increase the A-phase current i.sub.a :
The A-phase switching device Q2 is turned on, the switching devices Q3 and Q4 for the other phases are turned off, and the switching device Q1 is turned on and off in response to a PWM signal. Thereupon, when the switching device Q1 is on, the current i.sub.a flows through the common switching device Q1, A-phase coil (Ra; ZA), and A-phase switching device Q2 in the order named, and the voltage V is applied to the A-phase coil, so that the current i.sub.a flowing through the A-phase coil increases. When the switching device Q1 is turned off, on the other hand, energy accumulated in the A-phase coil causes the current i.sub.a to flow through the diode D1 of the common series circuit, A-phase coil (Ra; ZA), and A-phase switching device Q2 in the order named, and a voltage "0" is applied to the A-phase coil.
Thus, if the duty ratio of the PWM signal for turning on and off the common switching device Q1 is .eta.a, an average voltage applied to the A-phase coil, in the process of applying the positive voltage to the A-phase coil, is equal to the product of the duty ratio .eta.a and the main voltage V, that is, (.eta.a.times.V).
(2) When a negative voltage is applied to the A-phase coil to reduce the A-phase current i.sub.a :
In order to apply the negative voltage to the A-phase coil, all of the A-, B-, and C-phase switching devices Q2, Q3 and Q4 are turned off.
When the common switching device Q1, whose operating state is changed in response to the PWM signal, is on, the current i.sub.a flows through the common switching device Q1, A-phase coil (Ra; ZA), and A-phase diode D2 in the order named, and the voltage "0" is applied to the A-phase coil.
When the common switching device Q1 is off, on the other hand, the current i.sub.a flows through the diode D1 of the common series circuit, A-phase coil (Ra; ZA), and A-phase diode D2 in the order named, and a voltage "-V" is applied to the A-phase coil.
Thus, in the process of applying the negative voltage to the A-phase coil, the average voltage applied to the A-phase coil takes a value obtained by multiplying the difference between 1 and the duty ratio .eta.a by the product of the main voltage V and minus 1, that is, (1-.eta.a).times.(-V).
Through the operations (1) and (2) described above, the A-phase exciting current i.sub.a is controlled by means of the PWM signal so as to follow up a command current duping an A-phase excitation section. When the motor rotates so that the excitation phase changes to the B-phase, the switching devices Q2 and Q3 are turned off and on, respectively, which indicates only that the switching device Q3 serves in place of the switching device Q2 in the case of A-phase excitation described above. Thus, the operation of the switching devices Q1 and Q3 and the voltage applied to the B-phase coil have the same relationship as in the case of the A-phase. Likewise, when the motor rotates for C-phase excitation, the role of the switching device Q2 for the A-phase excitation is only replaced with that of the switching device Q4, and thus the operation and the voltage applied to the C-phase coil is substantially the same as in the case of the A- and B- phase coils.
FIGS. 4A and 4B are diagrams for illustrating the relationships between the coil currents i.sub.a, i.sub.b and i.sub.c for the individual phases, total current i.sub.t, and command current i.sub.cmd in this driver circuit.
In the case of the A-phase excitation, the A-phase current i.sub.a is controlled in accordance with the duty ratio .eta.a of the PWM signal, which is settled depending on a current deviation equivalent to the difference between the command current i.sub.cmd and the A-phase current i.sub.a, so as to rise and follow up the current command i.sub.cmd. When the excitation mode is changed from the A-phase excitation to the B-phase excitation in the next stage, the B-phase current i.sub.b rises and is controlled in accordance with a duty ratio .eta.b, which is settled depending on a current deviation equivalent to the difference between the command current i.sub.cmd and the detected B-phase current i.sub.b. However, the fall of the A-phase current immediately after the change to the B-phase excitation is not controlled at all.
Since the switching device Q2 is off immediately after the start of the B-phase excitation, the current (last-transition current) flowing through the A-phase coil flows through the common switching device Q1, A-phase coil, and A-phase diode D2 in the order named, and the voltage "0" is applied to the A-phase coil when the common switching device is on. When the common switching device is off, on the other hand, the current flows through the diode D1 of the common series circuit, A-phase coil, and A-phase diode D2 in the order named, and the "-V" is applied to the A-phase coil. As in the case of the aforesaid operation (2), therefore, the average voltage applied to the A-phase coil is given by (1-.eta.b).times.(-V).
The A-phase current i.sub.a is drastically reduced when the excitation phase is changed, and becomes 0 after the passage of a certain time (t.sub.ab). In the section t.sub.ab for the excitation phase change from the A-phase to the B-phase, the total current i.sub.t flowing through the motor is equal to the sum of the A-phase last-transition current and the B-phase first-transition current.
As mentioned before, however, the falling A-phase current i.sub.a is not controlled (because the average voltage for the A-phase is based not on the duty ratio .eta.a to be settled depending on the A-phase current deviation, but on the duty ratio .eta.b to be settled depending on the B-phase current deviation), although the B-phase current i.sub.b is controlled. Accordingly, the total current i.sub.t of the motor is not controlled for the section t.sub.ab. After the A-phase current i.sub.a becomes "0" when the section t.sub.ab terminates, the B-phase current i.sub.b is equivalent to the total current i.sub.t (=i.sub.b).
The same also applies to the cases of excitation phase changes from the B-phase to the C-phase and from the C-phase to the A-phase. As shown in FIGS. 4A and 4B, the total current i.sub.t of the motor is not controlled in specific sections t.sub.ab, t.sub.bc and t.sub.ca for phase changes.
In other sections than the excitation phase changing sections t.sub.ab, t.sub.bc and t.sub.ca, as described above, the total current i.sub.t is a current for each excitation phase, and this current is controlled so as to follow up the command current i.sub.cmd. In the phase changing sections t.sub.ab, t.sub.bc and t.sub.ca, however, only the first-transition current is controlled so as to follow up the command current i.sub.cmd, and the last-transition current is not controlled. After all, the total current i.sub.t of the motor is not controlled, resulting in a torque ripple. In FIG. 4B, a fine line is used to indicate that the total current i.sub.t of the motor is not controlled in sections t.sub.ab, t.sub.bc and t.sub.ca for changes, and therefore, cannot be securely made coincident with the command current i.sub.cmd.
In the case where the driver circuit of the VR motor is a driver circuit in which the number of switching devices used for the control is equal to (phase number+1), as described above, the current deviation equivalent to the difference between the command current i.sub.cmd and the detected current for the phase concerned is obtained for each phase, and the current for the phase concerned is controlled in accordance with the duty ratio for the phase concerned which depends on the current deviation. Accordingly, a current detector for detecting the current for each phase must be provided for each phase. Moreover, there are sections for the excitation phase changes in which the total current of the motor cannot be controlled, so that a torque ripple may be caused.